Indigobird males and females imprint on their host song during development (see Host-specificity). Males mimic host songs and incorporate them into their repertoire, and females use, at least in part, this mimicry to choose mates. Females then use song as a cue when choosing which nest to parasitize. These behaviors result in assortative mating among indigobird species based on host imprinting (Figure 1).

it also provides a mechanism for the rapid formation of new reproductively isolated populations after the colonization of new host species. If a female indigobird lays an egg in the nest of a novel host and if the resulting chick survives, it will learn the song of the new host and, in that single generation, will become reproductively isolated from its parent population. Parasitism of a novel host by several females in a local area might lead to the origin of a new indigobird species. Because females prefer males that mimic the song of their own foster species, indigobirds reared by a novel host species will form a reproductively isolated population. About 1 % of male indigobirds in southern Africa that were independently identified to species based on their morphology mimicked the song of a species that is not their usual host (Payne et al. 1993), suggesting that females do

Speciation by host shift

occasionally lay in the nests of different host species. Over the course of thousands or perhaps tens of thousands of years, it is not difficult to imagine this kind of colonization event (perhaps involving a number of females in a local area) occasionally giving rise to new, viable populations imprinted on new host species. Once established, these populations might evolve mimetic mouth markings relatively rapidly given their behavioral reproductive isolation from other indigobirds. Thus, indigobirds have apparently experienced a rapid and recent radiation precipitated by the colonization of new host species.

These same behavioral mechanisms, however, may also be responsible for hybridization between established indigobird species (Payne & Sorenson 2004). If a female lays in the nest of a host species associated with a different indigobird, then that offspring will mate with individuals of the other species. Thus, we are left with an intriguing, but incomplete picture of the speciation process in indigobirds. If extant indigobird species evolved recently and rapidly but are now essentially isolated, what factors led to this recent radiation at the tips of an ancient parasitic clade? On the other hand, if the high degree of genetic similarity among these species is due to a constant low level of genetic introgression, how are their morphological differences, particularly their mimetic mouth markings, maintained? Do indigobird “species” originate from multiple independent colonizations of the same host that later combine to form a single biological species? At present, to what extent are the species of indigobirds reproductively isolated?

Figure 1. Assortative mating among indigobirds based on song mimicry/imprinting.

The genetic data can be interpreted in light of what is known about indigobird behavior is as follows: While song learning and mimicry results in the cohesion of indigobird species and/or populations utilizing a particular host species (Balakrishnan & Sorenson 2006),

Figure 2. Model of how colonization of a novel host can lead to a reproductively isolated population.